1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and, more particularly, to a liquid crystal display in multiple alignment or MVA (multi-domain vertical alignment) mode in which liquid crystal molecules having negative dielectric anisotropy are aligned differently.
2. Description of the Related Art
Among a variety of flat panel displays, LCDs are regarded most promising displays that can replace CRTs. Further expansion of the market of LCDs is expected as a result of the application of them not only to display monitors of PCs (personal computers), word processors and office automation equipment but also to display units of consumer (home) electric apparatuses such as wide screen televisions and portable small televisions.
Presently, the most frequently used mode of display of LCDs is the normally white mode utilizing TN (twisted nematic) liquid crystals. Such an LCD has an electrode formed on each of surfaces of two glass substrate opposite to each other and horizontal alignment films formed on both of the electrodes. An alignment treatment is performed on the two horizontal alignment films in directions orthogonal to each other by means of rubbing or the like. On the outer surface of each of the substrates, there is provided a polarizer whose polarizing axis is aligned with the rubbing direction of the alignment film on the inner surface of the respective substrate.
When nematic liquid crystals having positive dielectric anisotropy are sealed between the substrates, liquid crystal molecules in contact with the alignment film are aligned in the rubbing direction. That is, the direction of alignment of the liquid crystal molecules in contact with the two alignment films are orthogonal to each other. As a result, the liquid crystal molecules between the two substrates are aligned in a direction perpendicular to the surfaces of the substrates with their directions of alignment sequentially rotated in planes in parallel with the substrate surfaces, and the liquid crystals are twisted between the substrate at an angle of 90 deg.
When light impinges upon the surface of one of the substrates of a TN-type LCD having the above-described structure, the polarizing direction of linearly polarized light which has passed through the polarizer on the substrate is rotated at an angle of 90 deg. along the twist of the liquid crystal molecules when it passes through the liquid crystal layer, and the light then passes through the polarizer on the other substrate which has a polarizing axis orthogonal to the polarizer on the first substrate. This makes it possible to provide display in a bright state when no voltage is applied (normally white mode).
When a voltage is applied between the electrodes opposite to each other, the twist is eliminated because the longitudinal axes of the nematic liquid crystal molecules having positive dielectric anisotropy are aligned in a direction perpendicular to the substrate surfaces. The liquid crystal molecules do not exhibit birefringence (birefringent anisotropy) against linearly polarized light incident upon the liquid crystal layer in such a state. Therefore, the incident light can not pass through the other polarizer because its polarizing direction is not changed. This makes it possible to provide display in a dark state when a predetermined maximum voltage is applied. By removing the voltage, the display can be returned to the bright state because of an aligning force. The inclination of the liquid crystal molecules can be controlled by varying the applied voltage to vary the intensity of light transmitted by the other polarizer, which makes it possible to perform gray-scale display.
Active matrix type TN TFT LCDs in which a TFT (thin film transistor) as a switching element for controlling a voltage applied between opposite electrodes is provided at each pixel are widely used in display monitors for PCs and portable televisions because of their low profiles, light weights and capability of allowing wide screens with high image quality. Techniques for manufacturing TN-type TFT LCDs have advanced dramatically in recent years, and they have become better than CRTs in terms of contrast and color reproducibility as viewed in front of the screen. However, TN-type TFT LCDs have a serious problem in that their viewing angles are small. In particular, the viewing angles are small in the vertical panel viewing directions. In one of the viewing direction, luminance in the dark state increases to show an image that is undesirably whitish. In the other direction, the display becomes too dark in general, and luminance inversion of an image occurs in halftones. This is the most serious problem of TN-type LCDs.
An LCD which has solved such a problem in the viewing angle characteristics of TN-type LCDs is the MVA-LCD disclosed in Japanese patent No. 2947350. Referring now to an example of the MVA-LCD, an electrode is first formed on each of opposite surfaces of two substrates facing each other at a predetermined interval. Vertical alignment films are formed on both of the electrodes, and liquid crystals having negative dielectric anisotropy are sealed between the two vertical alignment films. A plurality of linear protrusions made of an insulator are periodically formed between the electrodes on the substrates and the vertical alignment films. The linear protrusions facing each other between the two substrates are offset from each other by an amount equivalent to one half of their pitch as viewed in the normal direction of the substrate surfaces. The linear protrusions are used for alignment control to separate liquid crystals in a pixel region in a plurality of directions of alignment. The separation of alignment can be also achieved by providing slits on the electrodes instead of the linear protrusions.
Two polarizers whose polarizing axes are orthogonal to each other are provided on the outer surfaces of the two substrates. The mounting direction of the polarizers is adjusted such that the direction of the longitudinal axes of liquid crystal molecules which are tilted on the display surface of the substrates in response to application of a voltage are substantially at an angle of 45 deg. to the polarizing directions of the polarizers as viewed in the normal direction of the substrate surfaces.
When nematic liquid crystals having negative dielectric anisotopic properties are sealed between the substrates, the longitudinal axes of the liquid crystals are aligned in a direction perpendicular to the surface of the vertical alinment layers. As a result, liquid crystal molecules on the substrate surfaces are aligned perpendicularly, and liquid crystal molecules on tilted surfaces of the linear protrusions are aligned at an angle to the substrate surfaces.
When light impinges upon one of the substrates with no voltage applied between the two electrodes of the MVA-LCD having the above-described structure, linearly polarized light that has passed through the one of the polarizers to enter the liquid crystal layer travels along the longitudinal axes of the perpendicularly aligned liquid crystal molecules. The incident light travels with no change in the polarizing direction because no birefringence occurs in the direction of the longitudinal axes of the liquid crystal molecules and are absorbed by the other polarizer which has a polarizing axis orthogonal to the first polarizer. This makes it possible to provide display in the dark state (normally black mode) when no voltage is applied.
When a voltage is applied between the opposite electrodes, the longitudinal axes of the liquid crystal molecules are aligned in parallel with the substrate surfaces with the direction of alignment of the liquid crystal molecules on the substrate surfaces regulated in accordance with the direction of alignment of the liquid crystal molecules which are tilted in advance by the linear protrusions.
The liquid crystal molecules exhibit birefringent properties against light which enters the liquid crystal layer in this state, and the polarization of the incident light changes in accordance with the inclination of the liquid crystal molecules. Light which passes through the liquid crystal layer while the application of a predetermined maximum voltage becomes linearly polarized light with a polarizing direction rotated, for example, at an angle of 90 deg, and it can therefore be transmitted by the other polarizer to provide display in the bright state. The display can be returned to the dark state by an action of an aligning force, when the voltage is removed. The inclination of the liquid crystal molecules can be controlled by varying the applied voltage to vary the intensity of the light transmitted by the second polarizer, which makes it possible to perform gray-scale display.
In an active matrix type MVA TFT LCD having a TFT formed at each pixel, the direction of alignment of the liquid crystals in a pixel can be separated into a plurality of direction. This makes it possible to achieve a quite large viewing angle and high contrast compared to those of TN-type TFT LCDs. Since no rubbing process is required, the manufacturing steps can be simplified with an increase in the yield of manufacture.
However, conventional MVA type TFT LCDs are still to be improved in terms of the response time of display. Specifically, although they can achieve a high speed response when display is turned to black again after a change from black display to white display, they are somewhat inferior to TN-type TFT LCDs in terms of response time during a change from a certain halftone to another halftone.
Further, conventional MVA type TFT LCDs have optical transmittance which is about twice that of lateral field type IPS (In-plane switching) wide viewing angle LCDs but is not as good as that of TN-type TFT LCDs.
As described above, while MVA TFT LCDs have solved the problems with conventional LCDs in terms of the viewing angle, contrast and response time in displaying black, white and then black again, they are not still as good as conventional TN-type LCDs in terms of response time in displaying halftones and transmittance.
A description will now be made with reference to FIGS. 42A through 43C on reasons why the response of conventional MVA LCDs to halftones is slower than that of conventional TN-type LCDs. FIGS. 42A through 42C show schematic configurations of sections of an MVA LCD panel taken along in a direction perpendicular to the substrate surfaces thereof. FIG. 42A shows the alignment of the liquid crystals when no voltage is applied, and FIG. 42B shows the alignment of the liquid crystals when a voltage is applied. FIG. 42C is a conceptual diagram showing a state of alignment control. FIGS. 43A through 43C show schematic configurations of sections of a TN-type LCD panel taken along a direction perpendicular to the substrate surfaces. FIG. 43A shows the alignment of the liquid crystals when no voltage is applied, and FIG. 43B shows the alignment of the liquid crystals when a voltage is applied. FIG. 43C is a conceptual diagram showing a state of alignment control.
A TN-type LCD 100 will be first described with reference to FIGS. 43A through 43C. As shown in FIG. 43A, liquid crystals 102 of the TN-type LCD 100 are aligned at a twist of 90 deg. between an electrode 108 on an upper substrate 104 and an electrode 110 on a lower substrate 106 provided opposite to each other (alignment films on both of them are not shown) when not voltage is applied. When a voltage is applied between the electrodes 108 and 110, as shown in FIG. 43B, the liquid crystal molecules erect on the substrates 104 and 106 substantially perpendicularly thereto, which removes the twist. When the application of the voltage is stopped, the liquid crystal molecules rotate in a direction substantially in parallel with the surfaces of the substrates 104 and 106 to be in the twisted alignment again. As thus described, in the case of the TN-type LCD 100, it can be thought not only that the alignment of liquid crystal molecules in the vicinity of interfaces of the electrodes 108 and 110 to the alignment films (not shown) is controlled by a regulating force of the alignment films as indicated by the shaded part 112 in FIG. 43C but also that a twisted alignment achieved by adding a chiral agent or the like alignment control is achieved to some degree even on liquid crystal molecules located in the middle of the liquid crystal layer 102.
As shown in FIG. 42A, liquid crystal molecules except those located in the vicinity of linear protrusions 126, 128 and 130 among liquid crystals 124 of an MVA LCD 114 are aligned substantially perpendicularly to substrate surfaces between an electrode 120 on an upper substrate 116 and an electrode 122 on a lower substrate 122 (alignment films on both of the substrates are not shown) which are provided opposite to each other when no voltage is applied. Liquid crystal molecules in the vicinity of the linear protrusions 126 through 130 are aligned substantially perpendicularly to the surfaces of the alignment films which are not shown on inclined surfaces of the protrusions and are inclines relative to the surfaces of the substrates. When a voltage is applied between the electrodes 120 and 122, as shown in FIG. 42B, tilting of liquid crystals sequentially propagates in the tilting direction of the liquid crystal molecules in the vicinity of the linear protrusions 126 through 130 for regulating alignment. As a result, liquid crystal molecules in the middle of the region or gap between one linear protrusion and another linear protrusion arc tilted with a time lag. Especially, the speed of propagation of the tilting of liquid crystal molecules are low in the case of a change from black to a dark halftone because the amount of the change in the applied voltage is small and the change in the strength of electric fields in the liquid crystals is therefore also small.
The tilting directions of the liquid crystal molecules located between the gaps between the linear protrusions 126 through 130 are not defined unless the tilting directions are propagated from the linear protrusions 126 through 130. That is, the alignment of the liquid crystals in the MVA LCD is regulated only by distortion of electric fields in the vicinity of interfaces of the substrate surfaces to the alignment films on which a regulating force of the alignment films acts, on the alignment films on the linear protrusions 126 through 130 and in the vicinity of the same, as indicated by the shaded part 132 in FIG. 42C. The crystal alignment in other regions is controlled only indirectly.
The response time can be shortened even in the conventional MVA structure by reducing the distance of the gaps (pitch) between the linear projections on the upper and lower substrates. As described above, however, the tilting direction of liquid crystals of a normal MVA LCD is defined by inclined surfaces of protrusions made of an insulator. Therefore, the inclined regions must have a certain width, length and height. This places some limit on the reduction of the pitch of the upper and lower protrusions.
FIG. 44 shows the alignment of the liquid crystal molecules in the MVA LCD shown in FIGS. 42A through 42C when a voltage is applied as viewed from the lower substrate 118. The upper and lower protrusions 126 and 128 among the three linear protrusions 126 through 130 extending in the lateral direction of the figure are formed on the lower substrate 118, and the protrusion 130 in the middle is formed on the upper substrate 116.
As shown in FIG. 44, the alignment of the liquid crystal molecules which are aligned substantially perpendicularly to the surfaces of the substrates 116 and 118 when no voltage is applied is separated into an alignment region A in which they are aligned in the direction from the linear protrusion 130 on the upper substrate 116 toward the linear protrusion 128 on the lower substrate 118 (the direction of upwardly leaving the plane of the drawing) and an alignment region B in which they are aligned in the direction from the linear protrusion 130 toward the linear protrusion 126 on the lower substrate 118 (the direction of downwardly leaving the plane of the drawing).
Specifically, when a voltage is applied, alignment separation is performed on the liquid crystal molecules in the alignment regions A and B which are adjacent to each other with the linear protrusion 130 interposed therebetween such that the direction of the longitudinal axes of the liquid crystals in the alignment region A is substantially at an angle of +90 deg. to the linear protrusion 130 and such that the direction of the longitudinal axes of the liquid crystals in the alignment region B is substantially at an angle of −90 deg. to the linear protrusion 130. Liquid crystal molecules in the vicinity of the tops of the linear protrusions 126 through 130 are tilted in the extending direction of the protrusions when a voltage is applied and are aligned in a direction of alignment of about 0 or 180 deg. to (in parallel with) the linear protrusions 126 through 130.
As thus described, when a voltage is applied, the direction of alignment of the liquid crystal molecules in the display regions on the substrates 116 and 118 is rotated at an angle of 90 deg. to the direction of alignment of the liquid crystal molecules in the vicinity of the tops of the linear protrusions 126 through 130 (that is about 0 or 180 deg. to the linear protrusions 126 through 130). Therefore, liquid crystal molecules are arranged on both sides the inclined surfaces of the linear protrusions 126 through 130 in a direction of alignment of 45 deg. to the linear protrusions 126 through 130, as shown in FIG. 44. Meanwhile, the polarizing axes P and A of the polarizers indicated by two arrows orthogonal to each other in the figure are at inclined at 45 deg. relative to the direction of alignment of the liquid crystal molecules in the display regions A and B on the substrates 116 and 118.
Therefore, the direction of alignment of the liquid crystal molecules aligned in the direction of 45 deg. to the linear protrusions 126 through 130 are parallel and perpendicular to the polarizing directions of the polarizing axes P and A of the polarizers, respectively. As a result, two dark lines (disclination lines) 140 and 142 are generated on both sides of the inclined surfaces of the linear protrusions 126 through 130 as indicated by the broken lines in the figure. The two dark lines 140 and 142 are formed each of intervals between first singular points (indicated by (+1) in the figure) and second singular points (indicated by (−1) in the figure) in alignment vector fields formed on the linear protrusions 126 through 130. The longitudinal axes of the liquid crystal molecules are substantially directed toward the same point in the first singular points (+1), and a part of the liquid crystal molecules are aligned in a different direction at the second singular points.
When it is attempted to shorten the response of such a conventional MVA LCD to halftones by reducing the pitch of the upper and lower protrusions to form the protrusions with an increased density, an increase occurs not only in the area occupied by the protrusions in the pixel region but also in the density of the formation of the two dark lines 140 and 142 formed on both sides of the protrusions, which results in a reduction of transmittance at a degree that can not be ignored. Therefore, a problem arises in that a reduction of transmittance occurs when the density of the linear protrusions are formed in an increased density to improve the response characteristics of the liquid crystals. As thus described, the structure of a conventional MVA LCD has a problem in that an improvement of the response characteristics of the liquid crystals and an improvement of transmittance are in the relationship of trade-off.